(1) Field of the Invention
The present invention relates to a so-called "tandem-type" image forming apparatus where a plurality of image holding components, such as photosensitive drums, are set along the transportation path of a recording sheet.
(2) Related Art
Tandem-type image forming apparatuses have received much attention in recent years because of their ability to perform color printing at high speed.
A color copier, as one example of this type of image forming apparatus, has four photosensitive drums corresponding to four colors, i.e., cyan (C), magenta (M), yellow (Y), and black (K), set along the transportation path of a recording sheet. These photosensitive drums are made to rotate and are exposed to a scanning light beam. This forms an electrostatic latent image on the surfaces of the photosensitive drums. These electrostatic latent images are respectively developed into the corresponding color toner images. The toner images of cyan, magenta, yellow, and black respectively formed on the photosensitive drums are sequentially transferred onto a transported recording sheet. As a result, four color images are superimposed on the recording sheet to form a color image. Here, the mode where an image formation is performed with all of the photosensitive drums 41C to 41K being operative is referred to as the "full-color mode" hereinafter.
The photosensitive drums are rotated via the driving force of a motor. Suppose that a pulley which finally transmits the driving force to a photosensitive drum is mounted on the rotational shaft of each photosensitive drum. If the pulley is eccentric, the rotational speed of the photosensitive drums will fluctuate during each rotation of the pulley. In addition, if the speed fluctuation varies between photosensitive drums, color displacements occur in a color image formed on the recording sheet. To avoid this problem, there is a method whereby pulleys are respectively mounted on the rotational shafts of the photosensitive drums, with the eccentric directions being aligned. By means of this method, the fluctuation in the rotational speed of each photosensitive drum takes place at the same time, so that color displacements are prevented.
When performing operations aside from full-color image formation, such as when forming a black image using a tandem-type copier which is provided with a plurality of photosensitive drums, toner images are not formed on the photosensitive drums for cyan, magenta, and yellow, and a toner image is formed only on the photosensitive drum for black. Here, the mode where a black image formation is performed using only the photosensitive drum 41K used for forming a black image is referred to as the "monochrome mode" hereinafter.
For this reason, it is better for the photosensitive drums for C, M, and Y which are not used for forming the black image not to be rotated to prevent unnecessary wear and tear on these photosensitive drums and the cleaning blades that are in contact with them. However, if the photosensitive drum for K is only rotated while the photosensitive drums for C, M, and Y are not rotated, the eccentric direction of the pulley mounted on the rotational shaft of the photosensitive drum for K is misaligned with that of each pulley mounted on the rotational shafts of the photosensitive drums C, M, and Y. When a full-color image is formed in this state, color displacements occur in the transferred image.
Japanese Laid-Open Patent Application No. 5-197244 teaches a technique for realigning the eccentric directions of the pulleys that are misaligned during black image formation before full-color image formation is performed.
FIG. 1 shows a schematic construction of a driving mechanism for photosensitive drums provided in a copier disclosed in the cited Japanese Laid-Open Patent Application No. 5-197244.
As shown in FIG. 1, a gear 420 is integrally set on an output shaft 440 of a motor (not illustrated) at a lowest position. Relay shafts 320 and 330 are respectively set at the upper left and the upper right of the gear 420. A gear 400 is mounted on the relay shaft 320 via an electromagnetic clutch 390, while a gear 410 is integrally mounted on the relay shaft 330. The gears 400 and 410 respectively engage with the gear 420. Pulleys 350 and 360 are respectively mounted on the relay shafts 320 and 330 integrally.
Rotational shafts 300a, 300b, 300c, and 300d of the photosensitive drums for C to K (not illustrated) are set above the relay shafts 320 and 330. Pulleys 310a, 310b, 310c, and 310d are respectively mounted on the rotational shafts 300a to 300d. It is supposed that the eccentric directions of the pulleys 310a to 310d are aligned in FIG. 1.
Belts 370a, 370b, and 370c respectively run over the pulley 350 and the pulley 310a, the pulley 310b, and the pulley 310c. A belt 370d runs over the pulley 360 and the pulley 310d.
Interceptors 500 and 510 are respectively set on the pulley 310a and the pulley 310d at the positions shown in FIG. 1. Photo interrupters 520 and 530 are respectively set above the pulley 310a and the pulley 310d to respectively detect the interceptors 500 and 510.
When performing full-color image formation using a conventional copier which is provided with the stated driving mechanism, all of the photosensitive drums are rotated by turning on the electromagnetic clutch 390. Meanwhile, when performing the black image formation, the photosensitive drums for C, M, and Y are not rotated by turning off the electromagnetic clutch 390, so that only the photosensitive drum for K is rotated.
As a result, after black image formation is performed, the eccentric direction of the pulley 310d is misaligned with that of the pulleys 310a to 310c. In other words, the rotational angle of the pulley 310d measured between the photo interrupter 530 and the interceptor 510 is different from the rotational angle of the pulleys 310a to 310c measured between the photo interrupter 520 and the interceptor 500.
For this reason, before full-color image formation is performed, the motor (not illustrated) is activated after turning on the electromagnetic clutch 390. When the motor rotates at constant speed, a period of time .DELTA.t (referred to as the "time .DELTA.t" hereinafter) is measured between when one photo interrupter detects the corresponding interceptor and when the other photo interrupter detects the other corresponding interrupter. The time .DELTA.t is then compared with a predetermined allowable time. When the time .DELTA.t is within the predetermined allowable time, full-color image formation is performed. On the other hand, when the time .DELTA.t is not within the predetermined allowable time, the electromagnetic clutch 390 is turned off for a period of time corresponding to the time .DELTA.t. As a result, the eccentric directions of the pulleys 310a to 310d are aligned. In other words, the rotational angle of the pulley 310d that is measured from the photo interrupter 530 to the interceptor 510 can be made equal to the rotational angle of the other pulleys 310a to 310c measured form the photo interrupter 520 to the interceptor 500. Then, full-color image formation is performed. By doing so, color displacements in the full-color image formation can be prevented.
However, when a conventional copier, the processing for obtaining the time .DELTA.t needs to be performed before performing the full-color image formation to prevent the color displacements. As a result, the performance of the full-color image formation is delayed by the processing for obtaining the time .DELTA.t, meaning that it may take a long time to complete the full-color image formation.
In addition, although the three pulleys 531C to 531Y are respectively rotated by a driving force distributed from a single driving source, the conventional copier uses a driving force transmission mechanism that is realized by the belts. For this reason, there is also the risk that the eccentric directions of the pulleys 531C to 531Y may be misaligned due to skids between each belt and the corresponding pulley after an extended period of time of use.